The enclosed-lens retroreflective sheeting shown in FIG. 1 of U.S. Pat. No. 2,407,680 (Palmquist et al.) has in sequence a back reflector 10, a transparent spacing film 11 (more recently and here called a "space coat"), a monolayer of glass microspheres 13 partially embedded in a transparent binder coating 12 (more recently and here called a "bead-bond layer"), and a transparent covering 14 having a flat front face (more recently and here called a "top coat"). By having a flat front face, the top coat provides a number of advantages discussed at columns 16 and 17 of the patent, e.g., rain does not "black out" the reflex reflection. The top coat may be coated from solution as in Example 1 of the Palmquist patent, or it may be a preformed plastic film such as the polymethylmethacrylate film 26 of Example 1 of U.S. Pat. No. 4,367,920 (Tung et al.). The top coat of the retroreflective sheeting of FIG. 2 of the Palmquist patent consists of two layers 14 and 15. In retroreflective sheeting now on the market wherein the top coat consists of two layers, the outer layer is relatively hard to provide good resistance to abrasion, and the inner layer is softer to make the sheeting more conformable.
Although as sold, the top coat is usually at the surface of such retroreflective sheeting, purchasers usually apply over the top coat "lettering, symbols, designs, etc., by affixing thereto transparent colored films cut to the required shapes, or by painting with transparent colored paint; thereby forming an overlying transparent colored film or coating 16 indicated in FIG. 2, which acts as a colored filter . . . " (Palmquist patent, col. 11, lines 30-37).
While the retroreflective sheetings of FIGS. 1 and 2 of the Palmquist patent were built up from the back reflector 10, they may be made by an inverse procedure as described at the top of column 12 and illustrated in FIG. 3, building upon a top coat, there called a "flat-faced transparent front covering 17". First, a bead-bond layer is applied to the top coat, and then the glass microspheres are embedded in the surface of the bead-bond layer. Usually the microspheres are then covered with a space coat, and a thin-film reflective layer is depsoited onto the space coat. The exposed face of the reflective layer may then be adhesively bonded to a carrier.
Although the top coat of each of the examples of the Palmquist patent is an acrylic polyester, the top coat of almost all such retroreflective sheeting sold until the mid 1960's was an alkyd resin from vegetable oils. Unfortunately, fungus often pits the alkyd resin surface, especially when the humidity was high. Better resistance to fungus was achieved by substituting saturated polyesters for the vegetable oils, but this resulted in cracking during cold weather. Although this substitution reduced the yellowing experienced by alkyd resins, there continued to be a problem of yellowing from weathering.
While the acrylic polyesters of the Palmquist patent have good weather resistance and have been used for that reason, those that form a hard surface are quite brittle and hence tend to crase and crack while being applied. Softer acrylic polyesters are tougher, but are less resistant to weathering.
One reason alkyd resins have been used for top coats of retroreflective sheeting in spite of their weathering problems is that they have good resistance to solvents with which users like to clean the sheeting. Acrylic polyesters have poor solvent resistance. Another reason is that alkyd resins tend to afford better abrasion resistance than do acrylic polyesters.
Two examples of commercial retroreflective sheetings sold in recent years are as follows. In the first example, the top coat is believed to be two acrylic polyester layers, each of which is a hydroxy-functional acrylic polyol, namely, a mixture of ethyl acrylate, methyl methacrylate, and hydroxyethyl acrylate or methacrylate, which polyol has been cured with a methylated melamine resin. Retroreflective sheeting of this type is illustrated in FIG. 2 of U.S. Pat. No. 4,025,674 (Mizuochi). A different example uses a single-layer top coat which is a hydroxy-functional acrylic polyol cured with a melamine resin. While the top coats in both examples provide good solvent resistance, they provide only a compromise between toughness and weatherability. Hence, there has been a continuing demand for retroreflective sheeting, the top coat of which is as tough and as solvent resistant as alkyd resin and as weather-resistant as acrylic polyesters.
U.S. Pat. No. 3,190,178 (McKenzie) discloses retroreflective sheeting which is currently called "high intensity" or "encapsulated-lens" retroreflective sheeting. As illustrated in the patent, the retroreflective sheeting has a transparent cover film which is sealed to the face of the sheeting in a geometric pattern. Between narrow sealed areas are relatively broad unsealed areas wherein the front surfaces of the microspheres are optically exposed to an air interface. The patent says that a preferred transparent cover film is biaxially-oriented methyl methacrylate and lists other useful self-supporting transparent films at col. 5, lines 52-62. In such an encapsulated-lens retroreflective sheeting, the cover film serves as a top coat, but none of those named in the patent is as tough and as solvent resistant as alkyd resin and as weather-resistant as acrylic polyesters.
U.S. Pat. No. 3,689,346 (Rowland) discloses composite retroreflective sheeting in which minute, closely spaced cube-corner formations, are adhered to a separate film. The film serves as the "body portion" of the sheeting, and light rays entering the front surface of the film or body portion are reflected by the cube-corner formations. The body portion of this cube-corner retroreflective sheeting serves as its top coat, and preferred resins for the body portion are listed at col. 9, lines 24-27. Some of these have good solvent resistance, but none have good resistance to weathering. In other constructions a top coat can be applied over the body portion of a cube-corner retroreflective sheeting.